Iraqi-born Khamis Gewargis Khamis was questioned by the Australian Federal Police last week upon his return to Melbourne.
"Enquiries relating to his activities while overseas are ongoing," the Australian Federal Police spokesman said.
Khamis has joined an Assyrian Christian militia in northern Iraq known as Dwekh Nawsha and was intending to fight until Iraq is free from ISIL.
While in Iraq, Khamis confessed to an ABC reporter that he was concerned he might be breaking Australian anti-terrorism laws, but stressed he would face the law if needed.
According to the London-based International Centre for the Study of Radicalisation and Political Violence, between 100 and 250 Australian nationals are believed to be fighting in Iraq and Syria, including within the ISIL.
In February, Australian Prime Minister Tony Abbott announced that dual nationals involved in terrorism could have their Australian citizenship revoked. Since 2010, Canberra has canceled some 100 passports belonging to individuals with dual citizenship fighting in Syria and Iraq.